Method of nickel-gold plating and printed circuit board

ABSTRACT

Disclosed are a method of electroless nickel-gold plating an object and a printed circuit board. The method in accordance with an embodiment of the present invention includes: forming a first nickel plated layer on a surface of the object; forming a second nickel plated layer on the first nickel plated layer; and forming a gold plated layer on the second nickel plated layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.10-2008-0087865, filed with the Korean Intellectual Property Office onSep. 5, 2008, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to a method of nickel-gold plating and aprinted circuit board.

2. Description of the Related Art

In a typical printed circuit board, a copper (Cu) circuit is formed on acopper clad laminate (CCL) and then a nickel layer and a gold platedlayer are formed on the copper circuit before a final surface treatment.A nickel plated layer is used as a plating underlayer of the gold platedlayer and performs a function of preventing interdiffusion between thecopper and the gold plated layer. The gold plated layer intends toreduce an electrical resistance in the part of a substrate coming incontact with an electronic component and to improve bondability betweenthe substrate and the electronic component. The gold plating process isperformed in order of oil-removal, soft etching, catalyst, electrolessnickel plating and an gold plating.

With regard to the electroless plating, which is one of the methods oftreating the surface of the printed circuit board, the nickel platedlayer is plated by the electroless method using a reducing agent on acopper plated layer having circuits formed thereon. Then, the goldplated layer is formed on the nickel plated layer by a method ofsubstituting nickel with gold. Local corrosion occurs in a part of theplating underlayer due to a local cell effect generated during theprocess of the electroless plating. In case the local substitution isexcessive in the plating underlayer, a pinhole or discoloration of aplated surface can be observed after forming the electroless plating.Since such an excessive corrosion of the plating underlayer causes theplating underlayer to be diffused into the electroless plated surface, awire-bonding and a soldering effect, etc., are hereby deteriorated andthe reliability of the printed circuit board is reduced.

SUMMARY

The present invention provides a method of nickel-gold plating that hasa layered structure without causing a crack on a surface, and a printedcircuit board plated by using the method.

An aspect of the present invention features a method of electrolessnickel-gold plating an object. The method in accordance with anembodiment of the present invention can include: forming a first nickelplated layer on a surface of the object; forming a second nickel platedlayer on the first nickel plated layer; and forming a gold plated layeron the second nickel plated layer.

The forming of the first nickel plated layer can include: adhering acatalyst to the object; and performing electroless nickel plating on theobject. The forming of the second nickel plated layer can include:adhering a catalyst to the first nickel plated layer; and performingelectroless nickel plating on the first nickel plated layer. Thecatalyst can include palladium (Pd).

The method can further include before the forming of the first nickelplated layer, forming roughness on the surface of the object.

Another aspect of the present invention features a printed circuitboard. The printed circuit board in accordance with an embodiment of thepresent invention can include: a substrate; a circuit pattern formed ona surface of the substrate; a first nickel plated layer formed on thecircuit pattern; a second nickel plated layer formed on the first nickelplated layer; and a gold plated layer formed on the second nickel platedlayer. In this case, a thickness of the second nickel plated layer canbe less than a thickness of the first nickel plated layer.

A first palladium layer can be interposed between the circuit patternand the first nickel plated layer. A second palladium layer can beinterposed between the first nickel plated layer and the second nickelplated layer.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a process of forming a plating underlayer and agold plated layer in accordance with a conventional method ofnickel-gold plating.

FIG. 2 is an electronic microscope photograph showing a surface of aconventional nickel-gold plated layer.

FIG. 3 is an electronic microscope photograph showing a cross section ofa conventional nickel-gold plated layer.

FIG. 4 is a flowchart showing a method of nickel-gold plating inaccordance with an aspect of the present invention.

FIGS. 5 through 11 are views showing a method of nickel-gold plating inaccordance with an aspect of the present invention.

FIG. 12 is an electronic microscope photograph showing a surface of anickel-gold plated layer formed by a method of nickel-gold plating inaccordance with an aspect of the present invention.

FIG. 13 is an electronic microscope photograph showing a cross sectionof a nickel-gold plated layer formed by a method of nickel-gold platingin accordance with an aspect of the present invention.

FIG. 14 is a cross sectional view showing a printed circuit board inaccordance with another aspect of the present invention.

DETAILED DESCRIPTION

Since there can be a variety of permutations and embodiments of thepresent invention, certain embodiments will be illustrated and describedwith reference to the accompanying drawings. This, however, is by nomeans to restrict the present invention to certain embodiments, andshall be construed as including all permutations, equivalents andsubstitutes covered by the spirit and scope of the present invention. Inthe following description of the present invention, the detaileddescription of known technologies incorporated herein will be omittedwhen it may make the subject matter unclear.

Terms such as “first” and “second” can be used in describing variouselements, but the above elements shall not be restricted to the aboveterms. The above terms are used only to distinguish one element from theother.

The terms used in the description are intended to describe certainembodiments only, and shall by no means restrict the present invention.Unless clearly used otherwise, expressions in the singular numberinclude a plural meaning. In the present description, an expression suchas “comprising” or “consisting of” is intended to designate acharacteristic, a number, a step, an operation, an element, a part orcombinations thereof, and shall not be construed to preclude anypresence or possibility of one or more other characteristics, numbers,steps, operations, elements, parts or combinations thereof.

Hereinafter, embodiments of a method of nickel-gold plating and aprinted circuit board in accordance with the present invention will bedescribed in detail with reference to the accompanying drawings. Indescription with reference to accompanying drawings, the same referencenumerals will be assigned to the same or corresponding elements, andrepetitive descriptions thereof will be omitted.

A printed circuit board (PCB) uses a circuit made of copper (Cu) andperforms a function of connecting electrical signals between electroniccomponents mounted on a substrate. The PCB is a substrate designed tomount a semiconductor and has an upper surface connecting an Au wirewith the semiconductor and a lower surface to which a solder ballfunctioning as a lead is adhered. As the semiconductor has recently ahigher speed and a finer circuit, the printed circuit board is rapidlyand increasingly used as a package having a type for substituting anexisting lead frame.

With an electroless plating method among methods of treating the surfaceof the PCB, plating is performed through substitution reaction of acomponent to be plated among the plated underlayer and electrolessplating solution. FIG. 1 is a view showing a process of forming aplating underlayer and a gold plated layer in accordance with aconventional electroless plating method. A nickel plated layer 2 isplated by an electroless method using a reducing agent on a copperplated layer 1 on which a circuit is formed. Then, a gold plated layer 3is plated on the nickel plated layer 2 through a method of substitutingnickel with gold.

If local substitution in the nickel plated layer 2, i.e., a platingunderlayer is excessively performed during the process of electrolessgold plating, the local substitution may be a cause of a pinhole ordiscoloration of a plated surface, etc., after forming the gold platedlayer 3. The local excessive substitution in the plating underlayer canbe described with an electrochemical local cell effect. The platingunderlayer having ionization tendency higher than that of an electrolessplating component functions as an anode and can be easily oxidized intothe ion state of M2+, which can be described with typical Galvaniccorrosion model.

FIG. 2 is an electronic microscope photograph showing a surface of aconventional nickel-gold plated layer. FIG. 3 is an electronicmicroscope photograph showing a cross section of a conventionalnickel-gold plated layer. Nickel plating is performed forming a nodulefrom on the copper. As the nickel plated layer is thicker, the nodulegrows. Therefore, the nodule particle has a wider surface area and alarger size. The local substitution is performed along a relatively weaknodule boundary of the nickel plated layer. According to a conventionalnickel-gold plating method, since the thick nickel plated layer causesthe size of the nodule particle to be larger and the nodule boundary toconcentrate on a specified part thereof, corrosion is concentrated onthe specified part. Accordingly, as shown in FIGS. 2 and 3, it can benoted that a crack denoted by ‘A’ is formed on the gold plated layer dueto the local excessive substitution.

That is, in order to reduce the crack, a technology for reducing thesize of the nodule is required such that the nodule boundary is evenlydistributed without concentrating on a specified part thereof. Accordingto an embodiment of the present invention, the nickel plated layer isformed by two stages and the size of the nodule is prevented from beinglarger, so that the crack in the gold plated layer can be prevented.

FIG. 4 is a flowchart showing a method of nickel-gold plating inaccordance with an aspect of the present invention. FIGS. 5 through 11are views showing a method of nickel-gold plating in accordance with anaspect of the present invention. In FIGS. 5 through 11, illustrated arean object 10, a contaminant 11, a first palladium layer 15, a firstnickel plated layer 20, a second palladium layer 25, a second nickelplated layer 30 and a gold plated layer 40.

First, as shown in FIG. 5, the surface of an object 10 can be defatted(S100). Through this process, oil, a fingerprint and dust and the likeadhered to the surface of the object 10 are removed and water-absorptionis given to the surface of the object. Then, in the next step, thesurface of the object 10 is in better contact with an etching solutionso that chemical plating can be prevented from mis-adhesion due to airbubbles.

In the next step, as shown in FIG. 6, roughness can be formed on thesurface (S200). The surface of the object 10 is appropriately roughenedthrough soft etching, and then suitable adhesion strength and roughnessare given to a plating film. If the etching solution remains on theresin surface after the etching, chemical plating can not be completed,and thus the etching solution may be removed.

Next, a first nickel plated layer 20 is formed on the surface of theobject 10 (S300).

In order to form the first nickel plated layer 20 through electrolessnickel plating, a catalyst can be first adhered to the surface of theobject 10 as shown in FIG. 7 for the purpose of an effective platingprocess (S310). This is a process of absorbing a catalytic metal that isa core of the electroless plating on the resin surface. Palladium can beused as a catalyst. Palladium-tin (Pd—Sn) compound is adhered to thesurface of the object 10 and tin (Sn) salt is dissolved through anaccelerator process. Then, only metallic palladium remains on thesurface, and eventually a first palladium layer 15 is formed. Althoughthe first palladium layer 15 can physically cover the entire surface ofthe object 10, an amount of the first palladium layer that is adequateto function as a catalyst is needed. Accordingly, the first palladiumlayer 15 may be formed to have palladium metallic particles partlyscattered on the surface of the object 10.

After adhering the catalyst, an electroless nickel plating is performedon the surface of the object 10 (S320), and then a first nickel platedlayer 20 is formed. The plating is performed by extracting nickel metalonto the surface by using Palladium as a catalyst. The thicker the firstnickel plated layer is, the larger the nodule is (see FIG. 8).

If after forming the first nickel plated layer 20, the gold plating isimmediately performed on the first nickel plated layer 20, the largesize of the nodule in the first nickel plated layer creates the crack.Accordingly, one nickel plated layer is further formed in the presentinvention. That is, a second nickel plated layer 30 is formed on thefirst nickel plated layer 20 (S400). As shown in FIGS. 9 and 10, thesecond nickel plated layer 30 is formed by adhering the catalyst to thefirst nickel plated layer 20 (S410) and performing the electrolessnickel plating on the first nickel plated layer 20 (S420). Theseprocesses are similar to the process of adhering the catalyst on thesurface of the object 10 (S310) and the process of performing theelectroless nickel plating on the surface of the object 10 (S320)respectively.

A second palladium layer 25 is formed on the first nickel plated layer20 and new nodule particles are formed again from on the secondpalladium layer 25. As a result, the nickel plated layer 30 can beformed. It is possible to cause the nodule on the surface of the secondnickel plated layer 30 not to greatly grow by forming the second nickelplated layer 30 having a thickness thinner than that of the first nickelplated layer 20.

As shown in FIG. 11, a gold plated layer 40 is formed on the secondnickel plated layer 30 (S500). The nodule boundary the second nickelplated layer 30 is evenly distributed without concentrating on aspecified part thereof, so that the excessive substitution can beprevented. FIG. 12 is an electronic microscope photograph showing asurface of a nickel-gold plated layer formed by a method of nickel-goldplating in accordance with an aspect of the present invention. FIG. 13is an electronic microscope photograph showing a cross section of anickel-gold plated layer formed by a method of nickel-gold plating inaccordance with an aspect of the present invention. In FIGS. 12 and 13,it can be seen that the gold plated layer 40 is formed to have a uniformshape without cracks.

A printed circuit board according to another aspect of the presentinvention will be described. FIG. 14 is a cross sectional view showing aprinted circuit board in accordance with another aspect of the presentinvention. Illustrated are a substrate 5, a circuit pattern 10′, a firstpalladium layer 15, a first nickel plated layer 20, a second palladiumlayer 25, a second nickel plated layer 30 and a gold plated layer 40.

A substrate that is a base for a printed circuit board may be either amere insulation layer or a layer having a single layer or multiplelaminated layers. A circuit pattern formed on the substrate performs afunction of transferring an electrical signal, and is made of copper(Cu) having high electrical conductivity.

If gold plating 40 is performed on such a circuit pattern 10′, it ispossible to improve electrical characteristics, to prevent the corrosionof a chip mounting surface and to obtain bonding characteristics of asemiconductor package manufacturing process.

Because it is difficult to directly perform the gold plating on thecircuit pattern 10′ and the nickel plated layers 20 and 30 can be usedas plating underlayers, a layered structure is obtained, in which thegold plated layer 40 is formed on the nickel plated layers 20 and 30.Meanwhile, the local excessive substitution may be a problem asdescribed above, two nickel plated layers 20 and 30, i.e., the platingunderlayers are formed in order to prevent the local excessivesubstitution in the present invention.

In FIG. 14, illustrated is the second nickel plated layer 30 thinnerthan the first nickel plated layer 20. The second nickel plated layer 30is formed on the first nickel plated layer 20. The first nickel platedlayer 20 is formed on the circuit pattern 10′. In order to describe thelayered structure of the plated layer, the thickness of the plated layeris exaggerated as compared with the thickness of the plated layer to beactually formed. Since the second nickel plated layer 30 is thinner thanthe first nickel plated layer 20 and the nodule of the second nickelplated layer 30 grows slightly and stops, the gold plated layer 40 canbe formed without cracks, that is, local corrosion on the second nickelplated layer 30, as shown in FIG. 12.

A first palladium layer 15 can be interposed between the circuit pattern10′ and the first nickel plated layer 20. A second palladium layer 25can be interposed between the first nickel plated layer 20 and thesecond nickel plated layer 30.

The first palladium layer 15 and the second palladium layer 25 arecatalyst layers for forming the first nickel plated layer 20 and thesecond nickel plated layer 30. Since the thickness that physicallyseparates the layers may be smaller as long as it is thick enough tofunction as a catalyst, the first palladium layer 15 and the secondpalladium layer 25 may be shaped that palladium metal particles arepartly adhered to the circuit pattern or the first nickel plated layer20.

While the present invention has been described with reference to anembodiment thereof, it will be understood by those skilled in the artthat various changes and modification in forms and details may be madewithout departing from the spirit and scope of the present invention asdefined by the appended claims.

Numerous embodiments other than embodiments described above are includedwithin the scope of the present invention.

1. A method of electroless nickel-gold plating an object, the methodcomprising: forming a first nickel plated layer on a surface of theobject; forming a second nickel plated layer on the first nickel platedlayer, the second nickel plated layer being electroless nickel plated tobe thinner than the first nickel plated layer and formed with nodulesthat are smaller than a surface of the first nickel plated layer; andforming a gold plated layer on the second nickel plated layer.
 2. Themethod of claim 1, wherein the forming of the first nickel plated layercomprises: adhering a catalyst to the object; and performing electrolessnickel plating on the object.
 3. The method of claim 2, wherein thecatalyst comprises palladium (Pd).
 4. The method of claim 1, wherein theforming of the second nickel plated layer comprises: adhering a catalystto the first nickel plated layer; and performing electroless nickelplating on the first nickel plated layer.
 5. The method of claim 4,wherein the catalyst comprises palladium (Pd).
 6. The method of claim 1,further comprising, before the forming of the first nickel plated layer,forming roughness on the surface of the object.
 7. A printed circuitboard comprising: a substrate; a circuit pattern formed on a surface ofthe substrate; a first nickel plated layer formed on the circuitpattern; a second nickel plated layer formed on the first nickel platedlayer, the second nickel plated layer being formed to be thinner thanthe first nickel plated layer by electroless plating and formed withnodules that are smaller than a surface of the first nickel platedlayer; and a gold plated layer formed on the second nickel plated layer.8. The printed circuit board of claim 7, wherein a first palladium layeris interposed between the circuit pattern and the first nickel platedlayer.
 9. The printed circuit board of claim 7, wherein a secondpalladium layer is interposed between the first nickel plated layer andthe second nickel plated layer.